SD-interferometry and SD-OCT are technologies based on analyzing the spectrum of the interference signal produced between optical signal from an object under investigation and a local optical reference signal. OCT can produce in real time a cross section image of the object, i.e. a two dimensional (2D) image in the space (lateral coordinate, axial coordinate). SD methods can be implemented in two formats, as described in the article “Optical coherence tomography”, by A. Podoleanu, published in Journal of Microscopy, 2012 doi: 10.1111/j.1365-2818.2012.03619.x: (i) spectrometer based (SB) or (ii) by using a tunable laser or a swept source (SS).
OCT schematic diagrams implementing different SD-OCT modalities are presented in FIGS. 1 and 2. They consist in an optical source, 1, a Michelson interferometer, where an optical Splitter 2 and a Reference Mirror, 4, are used to produce a reference beam. A Microscopy Interface optics, 5, is employed to convey light from the Splitter 2 to, and from the Object 3 to be examined, up to a optical spectrum reader, 6, that performs spectral analysis of the interference of light between the reference beam returned by reference mirror 4 and the beam returned by the Object, 3, in the form of an electrical signal, 60, in relation to the spectrum of light at the interferometer output. The path traversed by the object wave from the splitter 2 to the object 3 and back represents the object path length, OPL. The path traversed by the reference wave from 2 to 4 and back represents the reference path length, RPL. An optical path difference (OPD) in the interferometer is defined as OPD=(OPL−RPL). The interface optics 5 contains a transversal scanning unit, 51, consisting in one or two transversal scanners, 511 and 512 to scan the object beam laterally over the object 3. The interference signal resulting from the superposition of the object and reference beams is filtered spatially in the interface optics 5 by other elements in 52, such as lenses, converging mirrors, pinhole (not detailed) before being sent to the optical spectrum reader unit 6.
For those skilled in the art it should be obvious that this is a generic scheme describing the principle of spectral domain interferometry (SDI) equipped with lateral scanning to perform OCT. In practice, fibre optics can be used, to convey light from 5 to 6, in which case the fibre performs the role of the pinhole 52. The splitter 2 can also be implemented in fibre. Also, a reference beam can be provided by light recirculation between optical splitters, as shown in FIG. 17. Obviously, other interferometers can be used as well.
Mechanical scanning of the OPD in time domain (TD)-OCT is replaced by reading the charges on the array in the spectrometer 61 in SB-OCT in FIG. 1 or by tuning the frequency of the laser source 12, in SS-OCT in FIG. 2.
2E points are sampled from the spectrum, either by using 2E pixels on the linear camera in the spectrometer, in case of the SB-SDI or by tuning the emission of the tuning source 12 in at least 2E resolvable spectral points in the case of SS-SDI.
The depth resolution is determined in both cases by the coherence length, cl, calculated by using the full width at half maximum (FWHM) of the spectrum bandwidth, Δλ, of the optical source 11 in SB-SDI or by using the tuning bandwidth Δλ of the SS 12 in SS-SDI, as cl˜λ2/Δλ, where λ is the central wavelength. Parameters cl and 2E are shown as determining the axial resolution and the axial range respectively of the A-scan so obtained in FIGS. 1 and 2. In both cases, 2EδX=Δλ and the axial range is proportional with another coherence length, cl˜λ2/δλ, where δλ is the bandwidth per photo-pixel of the camera in 61 or the line-width of the SS, 12.
The larger the modulus of OPD, the larger the number of peaks and troughs in the spectrum of the Michelson interferometer output, hence the reference to such spectrum as channelled, as shown in FIG. 3. Using 2E pixels, up to E cycles can be translated out of the signal 60. The optical spectrum reader 6 translates the channelled spectrum (CS), into an electrical signal 60. Irrespective of SB or SS method, the reading of the channelled spectrum at the output of the optical spectrum reader 6 delivers a signal of frequency proportional to the modulus of the OPD:f=U|OPD|  (1)where U is a conversion factor characterizing each SDI set-up.
SD-OCT refers to spectral interrogation of the spectrum at the interferometer output, ie of the CS signal. There are two possibilities, as illustrated in FIGS. 1 and 2. The operation of SD-OCT is based on the demodulation of the optical spectrum output of a low coherence interferometer. Inspecting the prior art in FIGS. 1 and 2, both SDI concepts, SB based SDI and SS based SDI, fit within the same structure, as sketched in FIG. 3(a)′. The spectral analysis of the interference spectrum at the output of the interferometer is performed having different elements in the source 1 and optical spectrum reader 6, either by using a broadband source, 11 in 1 and a spectrometer, 61 in 6 in the SB-SDI case in FIG. 1 or by tuning the optical source 12 in 1 and using a photo-detector 63 in 6, in the SS-SDI case in FIG. 2. The prior art executes spectral analysis using an FFT processor, 62. For the FFT operation to work properly, a calibrator, 620 is necessary to provide the channelled spectrum in equal slots of optical frequency, as explained below.
Spectrometer Based Optical Coherence Tomography (SB-OCT)
In FIG. 1, the optical source 1 is broadband, 11, the Processing Unit 6 employs a spectrometer, 61, usually built using a prism or a diffraction grating, and a linear photo-detector array, using a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) linear camera. Such method is referred to in what follows as spectrometer based (SB)-OCT. The spectrum exhibits peaks and troughs (channelled spectrum) and the period of such a modulation is proportional to the OPD in the interferometer, as shown in the article “Displacement sensor using channelled spectrum dispersed on a CCD array” published by Taplin et al., in Electron. Lett. 29, No. 10, (1993), pp. 896-897. The larger the OPD, the larger the number of peaks in the spectrum, S, 60, as shown in FIG. 3(a) when using a mirror as Object, 3, for two OPD values, for OPD of ˜3cl (left) and ˜6 cl (right). The linear camera in the spectrometer 61 needs pixels of sufficient small size, δλ, as shown in FIG. 3(b) to be able to sample the succession of peaks and troughs in the channelled spectrum. By downloading its charge content, the linear camera in the spectrometer transforms the optical spectrum into an electrical signal in time, as shown in FIG. 3(c). If multi-layered objects are imaged, such as retina or skin, each layer imprints its own spectrum modulation periodicity, depending on its depth.
The spectrum at the interferometer output, CS, is read in a time T by downloading the charge from the linear camera array, in 61. In doing so, an output signal, 60, is delivered by the Processing unit 6.
A block 62, FFT processor, provides fast Fourier transform (FFT) of the signal, 60, delivered by the linear camera in the spectrometer 61 and translates the periodicity of the signal CS, into peaks of different frequency, related to the OPD. Such a profile is essentially the A-scan profile of the square root of reflectivity in depth, signal 60′, as shown in FIG. 3(d) and at the bottom of FIG. 1, when the Object 3 is a mirror. For sensing applications this is the output of the measurement. For OCT applications, several A-scans for different transversal positions over the object 3, using 51, are required to assemble a cross section OCT image.
Swept Source Optical Coherence Tomography (SS-OCT)
In FIG. 2, the Processing unit 6 employs a Photo-detector, 63 and a Swept source (tuneable laser) 12 is used as optical source 1, operating according to a method referred to as swept source (SS)-OCT.
The illustration in FIG. 3 equally applies to SS-OCT, where Δλ and δλ are respectively the tuning bandwidth and line-width of the swept source 12. The signal 60 is in this case the temporal signal output of 6 when tuning the frequency of the swept source 12. In FIG. 2, the laser line, δλ, of the narrow band swept source 12, as shown in FIG. 3b, needs to be much narrower than the spectral distance between adjacent peaks in the channelled spectrum, as shown in FIG. 3a. FIG. 3c illustrates the signal output of the Photo-detector block 63 in FIG. 2, when tuning the SS 12. If in the ideal case, the laser line is approximated with a Dirac delta function (infinitesimally small line-width OX), then the photo-detected signal, 60, takes the exact shape of the channelled spectrum. A fast Fourier transform (FFT) of the signal 60 produced by 62 translates the periodicity of the channelled spectrum into peaks of different frequency, related to the OPD. In this way, an A-scan is obtained, as shown in FIG. 3(d) and at the bottom of FIG. 2, when the Object 3 is a mirror. The time required to tune the wavelength determines the time to produce an A-scan.
Flying Spot Versus Full Field Imaging
Each OCT method can admit different versions of scanning and detection. Any OCT system is equipped with two or three scanning mechanisms. Flying spot implementations use galvo-scanners, resonant scanners, piezo-elements and acousto-optic modulators as Scanning devices 511 and 512 in FIGS. 1 and 2 to deflect the beam over the Object, 3, point by point. Full field implementations use a 2D array, a CCD or CMOS camera, to capture several points in the scene at once. In FIG. 1, when operating in full field, the scanning device 51 between the splitter 2 and the object 3 is reduced to one scanner only, cylindrical optics is used in the Interface Optics 5 to illuminate the object 3 with a line and the camera in the spectrometer 61 is a 2D camera, as explained in “Line-field spectral domain optical coherence tomography using a 2D camera”, by J. Wang, C. Dainty, A. Gh. Podoleanu, published in Proc. SPIE 7372, 737221 (2009). A B-scan image is generated with no mechanical scanning, with lateral direction along one direction of the 2D camera in 61 (let us say along the rows) where for each pixel within the line projected on the object 3, the channelled spectrum is projected along the rectangular direction over the 2D camera (columns respectively). For each position of the lateral scanner 511, the 2D array delivers a cross section image (a B-scan), in the plane formed by the line projected on the object 3 and by the depth axis. Then the scanner 511 is moved to a new position to collect the next B-scan.
In FIG. 2, when operating in full field, the lateral scanning device 51 is removed, and the Pinhole in 52 and Photo-detector 63 are replaced with a 2D camera. In full field SS-OCT, processing is performed on each camera pixel to return an A-scan while tuning the Swept Source 12 in FIG. 2. In this way, the whole volume of the object is acquired with no mechanical scanning, as presented in “Evaluation of the signal noise ratio enhancement of SS-OCT versus TD-OCT using a full field interferometer”, published by J. Wang, M. Hathaway, V. Shidlovski, C. Dainty, A. Podoleanu, in Proc. SPIE 7168, 71682K (2009). Full field versions are compact solutions, however with the disadvantage of cross talk between pixels in the camera. Full field also provides an alternative for high speed acquisition in SS-OCT without the need of fast tuning rates for the SS. Access to high speed collection of 3D data can be either via increase in the sweep rate, or by combining a fast camera, such as a CMOS, with a slower SS, in a full field SS-OCT set-up, where the tuning speed is dictated by the frame rate of the camera in 63.
Therefore, the scanning elements in 51 and the optical elements in the interface optics 52 should be interpreted generically as covering al these different possibilities, where when flying spot is used, signal is received on a point photo-detector while in the full field case, repetitive deflection of the object beam is replaced with parallel reading of the interference signal by a 1D or a 2D array of photo-detectors. In such cases, the effect of the pinhole in 52 is now replaced, for each photo-site, by the small size aperture of each such photo-site and scanning device 51 moves in front of 6 (function taken by scanning the charge in photodetector arrays). So in the context of this disclosure, scanning means and detecting means should be interpreted generically as accomplishing the same function irrespective if mechanical scanners are used in the flying spot architecture or scanning the charge in linear array or 2D arrays of photo-detectors in the full field architecture.
Irrespective of the different principles of scanning used in SD-OCT, flying spot or full field, signal 60′ is produced in prior art via FFT in 62.
Resampling Problem
A first problem of the prior art, presented by SD interferometry is that the data coming from the spectrometer 61 in FIG. 1 and from the photo-detector block 63 (or camera in SS-full field implementations) in FIG. 2 is not provided linearly in optical frequency. This problem requires linearization of data, which takes time. In SB-OCT, the spectrum is not diffracted linearly in optical frequency over the linear camera, 61, used in the spectrometer. In SS-OCT, the variation of the optical frequency of the source 12 is not linear in time. For instance, many swept sources use a Fabry-Perot tunable filter. To achieve high line rate, these filters are excited with sinusoidal signals that leads to nonlinear variation in the frequency of the optical signal so generated. FFT of any data signal 60, which is not organized in linear optical frequency slots leads to smaller amplitude peaks, broader peaks, and to even multiple peaks in the final A-scan, signal 60′.
Therefore, specific FFT signal processing methods, linearization and calibration procedures have been developed and every prior art SD interferometer used in sensing and SD-OCT system uses extra devices, and extra procedures in a calibrator block, 620, to present the data to the FFT processor 62 in equal frequency slots. All these systems add extra cost and extra procedures take time and require significant computing resources.
SD-OCT has now reached over 1 MHz line scan rate, i.e. an acquisition of a spectrum can proceed that fast. However, the numerous steps of complicated real-time data processing procedures cannot be performed at these speeds. Numerical post-processing involves numerous steps, such as data resampling, numerical spectral shaping and apodization, Fourier transformation, and summation over parts of individual A-scans. These take time.
So far, several techniques have been demonstrated to calibrate the interferometric data for both SB-OCT and SS-OCT implementations.
Thus, in SB-OCT, a hardware optics method consists in placing a customized prism in the spectrometer, illustrated by the calibration block 620 in FIG. 1, which distributes the spectrum over the photo-detector array in the spectrometer linearly in optical frequency [Z. Hu and A. V. Rollins, “Fourier domain optical coherence tomography with a linear-in-wavenumber spectrometer,” Opt. Lett. 32, 3525-3527 (2007)]. Other methods are software based, such as using parametric iteration [B. Park, M. C. Pierce, B. Cense, Seok-Hyun Yun, M. Mujat, G. Tearney, B. Bouma, and Johannes de Boer, “Real-time fiber-based multi-functional spectral-domain optical coherence tomography at 1.3 μm,” Opt. Express 13, 3931-3944 (2005)] and phase linearization techniques [R. Leitgeb, W. Drexler, A. Unterhuber, B. Hermann, T. Bajraszewski, T. Le, A. Stingl, and A. Fercher, “Ultrahigh resolution Fourier domain optical coherence tomography,” Opt. Express 12, 2156-2165 (2004)]. In this case, calibration block 620 signifies all extra operations required, resampling, interpolation, linearization, zero padding, etc, performed over the data from the processing block 6 before being applied to the FFT processor 62.
Similarly, to compensate for the swept non-linearity in SS-OCT, several methods have been reported, such as: hardware approaches consisting in clocking the analog to digital converter with an electronic trigger-signal (k-clock) generated by a second interferometer [R. Huber, V. Wojtkowski, and J. G. Fujimoto, “Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography,” Opt. Express 14, 3225-3237 (2006), M. Gora, K. Karnowski, V. Szkulmowski, B. J. Kaluzny, R. Huber, A. Kowalczyk, and M. Wojtkowski, “Ultra high-speed swept source OCT imaging of the anterior segment of human eye at 200 kHz with adjustable imaging range,” Opt. Express 17, 14880-14894 (2009), J. Xi, L. Huo, J. Li, and X. Li, “Generic real-time uniform K-space sampling method for high-speed swept-Source optical coherence tomography,” Opt. Express 18, 9511-9517 (2010)] and hardware and/or software approaches consisting in optimizing the waveform applied to the tunable filter [C. Eigenwillig, B. Biedermann, G. Palte, and R. Huber, Opt. Express 16, 8916 (2008), Christoph V. Eigenwillig, Benjamin R. Biedermann, G. Palte and R. Huber, “K-space linear Fourier domain mode locked laser and applications for optical coherence tomography,” Opt. Express 16, 8916-8937 (2008), I. Trifanov, A. Bradu, L. Neagu, P. Guerreiro, A. Ribeiro, and A. G. Podoleanu, “Experimental Method to Find the Optimum Excitation Waveform to Quench Vechanical Resonances of Fabry-Perot Tunable Filters Used in Swept Sources,” Photon. Techn. Lett. 23, 825-827 (2011)]. Software approaches consist in re-sampling the data after the analogue-to-digital (A/D) conversion [S. Vergnole, D. Levesque, and G. Lamouche, “Experimental validation of an optimized signal processing method to handle non-linearity in swept-source optical coherence tomography,” Opt. Express 18, 10446-10461 (2010), Y. Yasuno, V. Dimitrova Vadjarova, S. Makita, M. Akiba, A. Morosawa, C. Chong, T. Sakai, Kin-Pui Chan, M. Itoh, and T. Yatagai, “Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments,” Opt. Express 13, 10652-10664 (2005), B. Chang Lee, M Yong Jeon, and T. Joong Eom, “k-domain linearization of wavelength-swept laser for optical coherence tomography,” Proc. SPIE 7894, Optical Fibers, Sensors, and Devices for Biomedical Diagnostics and Treatment XI, 789418, Feb. 16, 2011]. Unfortunately, all the methods mentioned above require either additional expensive equipment and/or are computationally expensive and limit the real time operation of the OCT systems.
All extra hardware devices, such as a clock in the swept source 12 driving a dual input digitiser for the signal from 63, or controller of the filter in the swept source 12 or software linearization techniques are included in an extra calibration block 620 in FIG. 2.
The block 620 either raises the cost of SD-OCT systems or slows down the image production. Even after applying linearization and calibration methods using the methods mentioned above, total compensation of nonlinearities is not achievable, irrespective of method, SB or SS.
Time to Produce an En-Face C-Scan Slice and Time to Collect a Volume in SD-OCT
A second problem of the prior art is that it cannot produce a 2D en-face map (C-scan OCT image) in real time. Therefore, prior art requires first to assemble the A-scans into a volume and second, produce software cuts in order to generate C-scans. SB-OCT and SS-OCT set-ups, irrespective of their versions, flying spot or full field, output A-scans, i.e reflectivity profiles along the axial coordinate, perpendicular to the C-scan plane. C-scans present the more familiar orientation as that provided by a microscope (transversal section to the on axis beam). C-scans provide enhanced visualization and additional information on tissue microstructure. They are also useful in the process of deciding where to sample the next high-resolution cross-section B-scan. C-scan sections can be obtained in SB-OCT and SS-OCT only after a whole volume of the Object 3 is acquired, i.e. via a post-acquisition process only. In prior art, in a first step, a series of B-scan OCT images is taken, at different transverse coordinates, Yv, with v=1, 2, . . . V, to sample the whole volume. This is followed by a second step, where the 3D volume so generated is sliced by software to obtain a C-scan. Therefore, in SD-OCT, the time to produce a C-scan is determined by the time required to collect all volume data, TV, plus the processing time necessary to assemble the A-scans into a volume and perform the software cut of such volume, Tcut. A 1 MHz line rate for instance allows the data for a B-scan image of 500 lines to be acquired in 500 microseconds. If 500 such frames of 500 pixels in depth in the A-scans are acquired, this means a volume of 5003 of pixel data captured in TV=0.25 s. This represents the minimum time interval to acquire the data necessary to produce a C-scan image. Extra time, TA is required to process the data, assemble the A-scans, generate a spatial volume of the sample and produce the, C-scan software cut in the volume, Tcut. In the prior art, Fourier transformations are used to generate axial reflectivity profiles (A-scans) from the reading of the output spectrum of the interferometer for every given pixel in transversal coordinate, (h, v), h=1, 2, . . . H, v=1, 2, . . . V. The volume is created from assembling together the A-scans for all H and V pixels along the X and Y coordinate respectively. Then, from such a volume, the corresponding en-face slice is software cut. This takes time.
In order to reduce the time for the en-face cut, a solution was proposed in the article “Real time en-face Fourier-domain optical coherence tomography with direct hardware frequency demodulation” published by B. R. Biedermann, W. Wieser, C. V. Eigenwillig, G. Palte, D. C. Adler, V. J. Srinivasan, J. G. Fujimoto, and R. Huber in Optics Letters, Vol. 33, Ho. 21/1, (2008), pp. 2556-2558. In this article, the amplitude of a single frequency band is extracted from the photo-detected signal while tuning the optical frequency of the optical source, by mixing the photo-detected signal with a signal of a particular chosen frequency delivered by a local oscillator. An en-face image contains points at the same axial position. This means that for the points in the en-face image, the same modulation of the channelled spectrum is produced. Points at the same OPD value produce the same number of peaks in the channelled spectrum and so when the channelled spectrum is read by tuning the optical frequency, a particular frequency is obtained for the pulsation of the photo-detected signal. However, this method also requires linearization and calibration of data. This method presents also the disadvantage that supplementary modulation of the swept source is needed to ensure a Gaussian profile for the final coherence gate. If more en-face images are required from more depths, then more filters or mixers need to be assembled in the digital interface. To produce a new en-face image at a different depth, the volume of data need to be read along the axial coordinate to produce the modulation corresponding to the depth wherefrom an en-face image is to be inferred from. If the calibration is imperfect, then the amplitude of the signal and the brightness in the image are lower.
Problem of Mirror Terms
Another disadvantage of the prior art SDI and SD-OCT methods is that the modulation of the channelled spectrum, as shown by equation (1) is the same for positive and negative values of the OPD. Several methods have been devised to recognise modulation for positive OPD values from modulation for negative OPD values, such as using a phase or a frequency modulator or by inserting a dispersing element in the interferometer. Such methods produce full axial range, i.e. allow utilisation of both signs of the OPD. Iterative numerical methods have been proposed in conjunction with a dispersing element along with methods to linearize the data before FFT, such as WO2010/007025A1 by Drexler W., Hover B., Povazay B., Matz G., “Method for image range extension in optical coherence tomography”. This patent application describes numerical procedures, which are taking long time to complete and as another disadvantage, they are capable of delivering a cross section image only. When en-face images are required, the time for processing is large, made from three components, time for full axial range reconstruction, time for resampling, interpolation/linearization/calibration, and time for volume construction and en-face cut.
Therefore, a need exists for processing methods in SDI, which do no require calibration or linearization in order to provide instantaneous signal for any given depth in the object, which translates in a need for systems using other functional blocks than Fourier transformation.
There is also a need for methods and systems to decode the channelled spectra faster in SDI applications and produce C-scan (en-face) images scans from different depths quicker in OCT applications.
A need also exists in terms of performing dispersion compensation faster.
A need exists for speeding up the signal processing in dispersive interferometers to provide quicker full axial range cross section imaging. In this respect, a need also exists to perform full axial range en-face imaging.
A need also exists for signal processing methods and devices more suitable to optical spectrometers that provide the spectrum reading in parallel.